In the oil and gas exploration and production industry, bores of ever increasing length are drilled to access subsurface hydrocarbon-bearing formations. For drilling through relatively hard rock it is known to incorporate hammer-drilling apparatus, such as the tool which has been supplied by the applicant under the ANDERHAMMER™ trade mark. A hammer-drilling apparatus is also described in applicant's U.S. Pat. No. 6,431,294, the disclosure of which is incorporated herein in its entirety. The ANDERHAMMER™ tool includes a reciprocating mass which is driven by fluid pressure to impact on an anvil coupled to a drill bit. The mass incorporates a piston which is cyclically exposed to drilling fluid pressure by operation of a rotating valve arrangement driven by a positive displacement motor. With the valve in one position, an upper face of the piston is exposed to upstream drilling fluid pressure and is urged downwards, into contact with the anvil. As the valve is rotated to a second position the drilling fluid is directed to bypass the piston, allowing a spring to return the mass and piston to an initial position. Use of the tool when drilling in hard rock, such as granite, has resulted in increased rates of penetration when compared to conventional drilling methods. Another reciprocating mass drilling tool is described in applicant's U.S. Pat. No. 7,461,706, the disclosure of which is incorporated herein in its entirety.
Other arrangements which may be useful in drilling are disclosed in applicant's U.S. Pat. Nos. 6,279,670, 6,508,317, 6,588,518, 6,439,318, the entire contents of which are incorporated herein in their entirety.